Atoms and Elements Chapter 2.

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2 Atomic theoryJohn Dalton, 1808: matter is made of tiny indestructible particles called atomsWhat evidence persuaded John Dalton that matter was made of atoms?

3 Evidence for Atoms Boyle’s Law (1660s)Gases can be compressed (PV = constant)Suggests that a gas is made of particles with space betweenConservation of mass (Lavoisier, 1789)In a chemical reaction, matter is neither created nor destroyed

4 Evidence for Atoms Law of Definite Proportions (Proust, 1797)All samples of a compound, regardless of source or how prepared, have the same proportions of their constituent elements (constant composition)Law of Multiple Proportions (Dalton, 1803)When two elements (A and B) form two different compounds, the masses of B that combine with 1 g of A can be expressed as a ratio of small whole numbers.

6 Evidence for Atoms Combining Volumes of GasesWhen two gases combine to form a new compound, the volumes that combine will be in a ratio of small whole numbers.

7 A New System of Chemical Philosophy: John Dalton, 1808Each element is composed of tiny indestructible particles called atoms.All atoms of a given element have the same mass and other properties that distinguish them from the atoms of other elements.Atoms combine in simple, whole-number ratios to form compounds.Atoms of one element cannot change into atoms of another element. In a chemical reaction, atoms change the way that they are bound together with other atoms to form a new substance.

14 The gold foil experimentErnest Rutherford (1910)Bombarded gold foil with alpha particlesMost went straight throughSome slightly deflectedA few strongly deflected“about as credible as if you had fired a 15-inch shell at a piece of tissue paper, and it came back and hit you”

16 Rutherford’s nuclear modelMass & positive charge concentrated in nucleusMost  particles miss the nucleus & are not deflected: most of the atom is empty spaceSome  particles come near the nucleus & are deflected: nucleus is positively charged and very denseTiny, lightweight electrons circle the nucleus, like planets around the sun

19 Ion chargeWhen an atom loses or gains an electron, it becomes an ionIon charge is shown in the upper right corner of the atomic symbolIf no charge is shown, the charge is zeromass number, Aprotons + neutrons2311Na1+ion chargeprotons – electronsatomic number, Zprotons

22 IsotopesAtoms with the same number of protons but different numbers of neutrons are isotopes of the same element6Li and 7Li are isotopes of lithiumBoth are the element lithium6Li has 3 protons, 3 neutrons7Li has 3 protons, 4 neutrons

32 A unit for counting atomsWe have units for describing mass, volume, temperature, and so forth, butwe need a unit for counting the number of itemsFor large items like eggs or donuts, we can use dozen: one dozen = 12 itemsFor tiny items like atoms or molecules, we need the mole!

33 What is a mole?One mole is defined as the number of atoms in exactly 12 grams of 12COne mole contains x 1023 particlesThis number is called Avogadro’s number (NA)There is Avogadro’s number of particles in a mole of any substance

34 Calculations with AvogadroHow many atoms of gold are present in mol Au?How many moles of Pb atoms are 8.27 x 1022 atoms of Pb?

38 Avogadro and molar massWhat is the mass of 2.35 x 1024 atoms of Cu?

39 Avogadro and molar massHow many He atoms are present in a 22.6 g sample of He gas?

40 Mole concept Examples 2-7A, 2-7B, 2-8A, 2-8BHow many atoms of gold are present in 5.07 x 10–3 mol Au?How many 206Pb atoms are present in 8.27 x 10–3 mol of Pb? (Pb contains 24.1% 206Pb )What is the mass of 2.35 x 1024 atoms of Cu?How many He atoms are present in a 22.6 g sample of He gas?

41 Mole concept Examples 2-9A and 2-9 BHow many Pb atoms are in a piece of lead with volume cm3? Density of Pb = g/cm30.100 mg of Re contains 2.02 x 1017 atoms of 187Re. What is the percent abundance of 187Re in this sample?

43 Exercise 31In one experiment, 2.18 g sodium were reacted with g chlorine. All the sodium was used up, and 5.54 g sodium chloride were produced. In a second experiment, 2.10 g chlorine were reacted with g sodium. All the chlorine was consumed, and 3.46 g sodium chloride were produced. Are these results consistent with the law of constant composition?

44 Exercise 38 In an experiment like the oil drop experiment,drop 1 carried a charge of 6.41 x 10–19 Cdrop 2 had 1/2 the charge of drop 1drop 3 had twice the charge of drop 1drop 4 had a charge of 1.44 x 10–18 Cdrop 5 had 1/3 the charge of drop 4Do these data support the value of x 10–19 C for the electron charge? Could Millikan have inferred the electron charge from these data?

45 Exercise 42 For the ion 228Ra2+, with a mass of 228.030 u, determineThe numbers of protons, neutrons, and electronsThe ratio of the mass of this atom to that of an atom of 16O ( u)

46 Exercise 49 There are three naturally occurring isotopes of magnesium:Calculate the weighted average atomic mass of magnesium.

47 Exercise 54Boron has a weighted average atomic mass of u. What are the percent natural abundances of its two isotopes, 10B ( u) and 11B ( u)?

48 Exercise 58 Determine The number of Ar atoms in 5.25 mg argonThe molar mass of an element if the mass of 2.80 x 1022 atoms of that element is 4.24 gThe mass of a sample of aluminum that contains the same number of atoms as g zinc

49 Exercise 60What is the total number of atoms present in a 75.0 cm3 sample of plumber’s solder, an alloy containing 67% Pb and 33% Sn by mass and having a density of 9.4 g/cm3?

50 Early Observations of Mass 1770sLavoisier - Conservation of MatterCareful measurements showed that mass is neither created nor destroyed in a chemical reaction.The total mass before and after reaction is the same

51 Conservation of mass Examples 2-1A and 2-1BA g sample of magnesium reacts with g nitrogen gas. The sole product is magnesium nitride. After the reaction the mass of unreacted nitrogen is g. What mass of magnesium nitride was produced?A 7.12g sample of magnesium is heated with 1.80 g of bromine. All the bromine is consumed and 2.07 g of magnesium bromide is the only product. What mass of magnesium is left over?

52 Early Observations of Matter 1799Proust - Constant CompositionAll samples of a given compound contain exactly the same proportion of elements by mass.This is the Law of Definite Proportions

53 Constant composition Examples 2-2A and 2-2BThis information was provided in the example:A g sample of magnesium combines with oxygen to yield g magnesium oxideWhat is the mass of magnesium in g of magnesium oxide?You wish to make exactly 2.00 g of magnesium oxide. What masses of magnesium and oxygen must you combine?

54 Early Observations of Matter 1803John Dalton - Law of Multiple ProportionsWhen the same elements form different compounds, the combining ratios in the compounds are always in a whole number relationship.Compound II has exactly twice as much O per C as compound I, by massmass of Oxygenreacting w/ 1 g of Carboncmpd I g O/g Ccmpd II g O/g C

63 Atomic composition Examples 2-3A and 2-3BWrite an appropriate symbol for the species with 47 protons, 61 neutrons, and 47 electronsDetermine the numbers of protons, neutrons, and electrons in an ion of sulfur-35 that carries the charge 2–

66 Periodic TableDmitri Mendeleev developed the modern periodic table in 1869,based on his belief that element properties are periodic functions of their atomic massesWe now know that element properties are periodic functions of their ATOMIC NUMBERS